Detection of chemical species in air, such as industrial pollutants, poisonous gases, chemical fumes, and volatile organic compounds (VOCs), is vital for the health and safety of communities around the world (see Watson J and Ihokura K (1999) Special issue on Gas-Sensing Materials, Mater. Res. Soc. Bull. 24:14). The development of reliable, portable gas sensors that can detect harmful gases in real-time with high sensitivity and selectivity is therefore extremely important (Wilson D M et al. (2001) “Chemical Sensors for Portable, Handheld Field Instruments,” IEEE Sensors Journal 1:256-274; Eranna G et al. (2004) “Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review/Integrated Gas Sensors—A Comprehensive Review,” Critical Reviews in Solid State and Material Sciences 29:111-188).
Due to their small size, ease of deployment, and low-power operation, solid-state thin film sensors are favored over analytical techniques such as optical and mass spectroscopy, and gas chromatography for real-time environmental monitoring (Wilson D M et al. (2001), supra, IEEE Sensor Journal 1:256-274; Shimizu Y and Egashira M (1999) “Basic Aspects and Challenges of Semiconductor Gas Sensors,” Mater. Res. Soc. Bull. 24:18; Sze S M (1994) Semiconductor Sensors 1st ed, Willey; New York). Selectivity, which is a sensor's ability to discriminate between the components of a gas mixture and provide detection signal for the component of interest, is an important consideration for the sensor's real-life applicability. Conventional metal-oxide based thin film sensors, despite decades of research and development (Brattain J B W H (1952) “Surface properties of germanium,” Bell. Syst. Tech. Journal 32:1; Azad A M et al. (1992) “Solid-State Sensors: A Review,” J. Electrochem. Soc. 139(12):3690-3704), still lack selectivity for different species and typically require high working temperatures (Meixner H and Lampe U (1996) “Metal oxide sensors,” Sens. and Actuators B 33:198-202; Nicoletti S et al. (2003) “Use of Different Sensing Materials and Deposition Techniques for Thin-Film Sensors to Increase Sensitivity and Selectivity,” IEEE Sensors Journal 3:454-459; Demarne V and Sanjines R (1992) Gas Sensors-Principles, Operation and Developments ed. G. Sberveglieri, Kluwer Academic, Netherlands). As such, the usability of such conventional sensors is severely limited and poses long-term reliability problems.
For a chemical sensor, the active surface area is an important factor for determining its detection limits or sensitivity. It is known that the electrical properties of nanowires (NWs) change significantly in response to their environments due to their high surface to volume ratio (Cui Y et al. (2001), supra, Science 293:1289-1292; Zhang D et al. (2004) “Detection of NO2 down to ppb levels using individual and multiple In2O3 nanowire devices,” Nano. Lett. 4:1919-1924; Kong J et al. (2000) “Nanotube Molecular Wires as Chemical Sensors,” Science 287:622-625; Comini E et al. (2002) “Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts,” Appl. Phys. Lett. 81:1869). NWs are therefore well suited for direct measurement of changes in their electrical properties (e.g. conductance/resistance, impedance) when exposed to various analytes. Substantial research has demonstrated the enhanced sensitivity, reactivity, and catalytic efficiency of the nanoscale structures (Cui Y et al. (2001), supra, Science 293:1289; Li C et al. (2003) “In2O3 nanowires as chemical sensors,” Appl. Phys. Lett. 8:1613; Wan Q et al. (2004) “Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors,” Appl. Phys. Lett. 84:3654; Wang C et al. (2005) “Detection of H2S down to ppb levels at room temperature using sensors based on ZnO nanorods,” Sens. and Actuators B 113:320-323; Wang H T et al. (2005) “Hydrogen-selective sensing at room temperature with ZnO nanorods,” Appl. Phys. Lett. 86:243503; Raible I et al. (2005) “V2O5 nanofibers: novel gas sensors with extremely high sensitivity and selectivity to amines,” Sens. and Actuators B 106:730-735; McAlpine M C et al. (2007) “Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors,” Nat Mater 6:379-384).
There have been attempts to demonstrate sensors based on nanotube/nanowire decorated with nanoparticles of metal and metal-oxides. For example, Leghrib et al. reported gas sensors based on multiwall carbon nanotubes (CNTs) decorated with tin-oxide (SnO2) nanoclusters for detection of NO and CO (see Leghrib R et al. (2010) “Gas sensors based on multiwall carbon nanotubes decorated with tin oxide nanoclusters,” Sens. and Actuators B: Chemical 145:411-416). Using mixed SnO2/TiO2 included with CNTs, Duy et al. demonstrated ethanol sensing at a temperature of 250° C. (Duy N V et al. (2008) “Mixed SnO2/TiO2Included with Carbon Nanotubes for Gas-Sensing Application,” J. Physica E 41:258-263). Balázsi et al. fabricated hybrid composites of hexagonal WO3 powder with metal decorated CNTs for sensing NO2 (Balázsi C et al. (2008) “Novel hexagonal WO3 nanopowder with metal decorated carbon nanotubes as NO2 gas sensor,” Sensors and Actuators B: Chemical 133:151-155). Kuang et al. demonstrated an increase in the sensitivity of SnO2 nanowire sensors to H2S, CO, and CH4 by surface functionalization with ZnO or NiO nanoparticles (Kuang Q et al. (2008) “Enhancing the photon-and gas-sensing properties of a single SnO2 nanowire based nanodevice by nanoparticle surface functionalization,” J. Phys. Chem. C 112:11539-11544). ZnO NWs decorated with Pt nanoparticles were utilized by Zhang et al., showing that the response of Pt nanoparticles decorated ZnO NWs to ethanol is three times higher than that of bare ZnO NWs (Zhang Y et al. (2010) “Decoration of ZnO nanowires with Pt nanoparticles and their improved gas sensing and photocatalytic performance,” Nanotechnology 21:285501). Chang et al. showed that by adsorption of Au nanoparticles on ZnO NWs, the sensor sensitivity to CO gas could be enhanced significantly (Chang S-J et al. (2008) “Highly sensitive ZnO nanowire CO sensors with the adsorption of Au nanoparticles,” Nanotechnology 19:175502). Dobrokhotov et al. constructed a chemical sensor from mats of GaN NWs decorated with Au nanoparticles and tested their sensitivity to N2 and CH4 (Dobrokhotov V et al. (2006) “Principles and mechanisms of gas sensing by GaN nanowires functionalized with gold nanoparticles,” J. Appl. Phys 99:104302). GaN NWs coated with Pd nanoparticles were employed for the detection of H2 in N2 at 300K by Lim et al. (Lim W et al. (2008) “Room temperature hydrogen detection using Pd-coated GaN nanowires,” Appl. Phys. Lett. 93:072109).
Although such results demonstrate the potentials of the nanowire-nanocluster based hybrid sensors, fundamental challenges and deficiencies in such prior attempts remain. Most of the results provide for mats of nanowires. Although such mats may increase sensitivity, the complex nature of inter-wire conduction makes interpreting the results difficult. Also, room-temperature operation of such previous sensors has not been demonstrated, and the selectivity is shown for only a very limited number of chemicals. Conventional sensor devices require high operating temperatures (≧250° C.) and large response times (more than 5 minutes). Indeed, such temperature-assisted sensors typically provide for an integrated heater for the device. Further, the reported sensitivities of such conventional devices were quite low even with long response times. Further, such conventional devices typically do not provide for air as the carrier gas. However, the ability of a sensor to detect chemicals in air is what ultimately determines its usability in real-life.
Thus, such demonstrations have resulted in poor selectivity of known chemical sensors, and therefore have not resulted in commercially viable gas sensors. For real-world applications, selectivity between different classes of compounds (such as between aromatic compounds and alcohols) is highly desirable. For example, the threat of terrorism and the need for homeland security call for advanced technologies to detect concealed explosives safely and efficiently. Detecting traces of explosives is challenging, however, because of the low vapor pressures of most explosives (Moore, D S (2004) “Instrumentation for trace detection of high explosives,” Review of Scientific Instruments 75(8):2499-2512; Yinon J (2002) “Field detection and monitoring of explosives,” TrAC Trends in Analytical Chemistry 21(4):292-301; Senesac L. and Thundat T G (2008) “Nanosensors for trace explosive detection,” Materials Today 11(3):28-36. Moreover, the difficulty of explosive detection is aggravated by the noisy environment which masks the signal from the explosive, the potential for high false alarms, and the need to determine a threat quickly. As such, trained canine teams remain the most reliable means of detecting explosive vapors to date; however, dogs are expensive to train and tire easily.
An ideal chemical sensor would be able to distinguish between the individual analytes belonging to a particular class of compounds, e.g. detection of the presence of benzene or toluene in the presence of other aromatic compounds, detection of a particular explosive compound, detection of a particular alcohol, etc. This is extremely challenging as most semiconductor-based sensors use metal-oxides (such as SnO2, In2O3, ZnO) as the active elements, which are limited due to the non-selective nature of the surface adsorption sites. The surface/adsorbate interactions of conventional sensor structures are limited and non-specific. Thus, conventional sensor devices lack the same selectivity as their bulk-counterpart devices.
Accordingly, there is a need for a nanostructure sensor device that solves one or more of the deficiencies of conventional devices.